The present invention relates to a transmitting device, in particularly a high-frequency transmitting device, for use in a mobile radio system, for example.
A conventional high-frequency transmitting device in accordance with the prior art is shown, by way of example, in FIG. 3. This high-frequency transmitting device includes a transmit signal generator 1 for generation of a modulated high-frequency (HF) signal that is applied to a transmitter amplifier 2. In high-frequency transmitters, particularly in mobile radio transmitters, part of the output power generated by the transmitter amplifier 2 is frequently decoupled in order to control the output power; i.e., the amplification of the transmitter amplifier 2, or the modulation effected by the transmitted signal generator 1. For this purpose, a device 3 for decoupling part of the output power generated by the transmitter amplifier 2 is provided, as shown in FIG. 3, in order to apply the part of the output power decoupled in this way to the transmitted signal generator 1 for power or modulation control.
The power or modulation control can, however, be disturbed by interference signals that are mixed with the decoupled or feedback signals from the device 3. These interference signals can derive both from reflected transmitter power and from other transmission sources 5, and they consist of signals received via the antenna 4. The magnitude of the interference power relative to the decoupled transmitter power is generally decisive for the interference effect, so that the interference susceptibility is highest where the transmitter power is lowest.
Generally, a so-called directional coupler is used as the device 3 in order to ensure that the signal applied to the transmitted signal generator 1 particularly includes the wave passing from the transmitter amplifier 2 to the antenna 4, but not the wave coming from antenna 4. The directional effect of this directional coupler 3 can significantly improve the interference immunity to reflections and radiations from external sources 5, provided the interference signal coming from the antenna 4 is not reflected from the transmitter amplifier 2 and again passes through the directional coupler 3, as indicated by arrows A and B in FIG. 3. In this case arrow A represents the interference signal made up of the reflected transmitter power and the transmitted signal originating from the antenna 5 and received via antenna 4, whereas arrow B designates the interference signal then reflected from the transmitter amplifier 2, decoupled from the directional coupler 3 and finally applied to the transmitted signal generator 1. The reflection of the interference signal coming from antenna 4, already described and shown in FIG. 3, at the output of the transmitter amplifier 2 is, however, generally of a high degree because there is no impedance matching. As such, the interference immunity of the high-frequency transmitter can only be slightly improved by using a directional coupler 3.
For this reason, the use of non-reciprocal components 6, such as insulators, in the transmitted signal path was proposed to attenuate the interference signal coming from the antenna 4. These non-reciprocal components 6 have a directional-dependent attenuation that differs greatly so that the signal direction from the antenna 4 to the transmitter amplifier 2 is blocked. Non-reciprocal components or isolators 6 of this kind nevertheless have a residual attenuation in the forward direction (i.e., in the direction from the transmitter amplifier 2 to the antenna 4), that results in an increase in the transmitter power to be applied from the high frequency transmitter and also in the cost and space requirement of these components. Furthermore, insulators are generally narrowband components.
Document EP-A-936 745 discloses a transmitting device whereby a transmitted signal generated by a transmitted signal generator is applied to a transmitter amplifier to amplify the transmitted signal. Furthermore, a circuit is provided that has a specific impedance and can be coupled to the output of the transmitter amplifier
Document EP-A-741 463 discloses a transmitting device with a signal generator for generation of a transmitted signal, a transmitter amplifier for amplification of the transmitted signal, connected after the transmitted signal generator, together with a sampler downstream of the transmitter amplifier that is connected to an output power detection device, and a circulator connected after the sampler.
An object of the present invention is, therefore, to provide a transmitter device in accordance with the generic kind that provides a sufficiently high interference immunity without also using non-reciprocal components.